The manufacturing of reflective VBG filters for a wide variety of wavelengths critically depends on the ability to record these filters holographically using a fixed recording wavelength λrec that is shorter than the operation wavelength λop of the filter. Accordingly, a method of “side-writing” a VBG filter was developed for photorefractive lithium niobate. Such a method is described in U.S. Pat. No. 5,491,570, for example.
This method has a number of drawbacks, such as, for example, the limited usable wavelength range (i.e., λop>n*λrec), complexity of sample preparation (e.g., the necessity to polish at least two orthogonal faces of the sample), and the inability to tune the operating wavelength in a wide range (i.e., greater than approximately 40 nm). Further, the method of “side-writing” has a fundamental limit on the clear aperture of the filter recorded in this way. This is due to the fact that the recording beams of light are necessarily absorbed in the material in order to create the required photo-induced changes of the refractive index and, as a result, the penetration depth of the recorded grating is limited by the material absorption. For this reason, the clear aperture of reflective VBG filters recorded in this way is typically no more than approximately 4-6 mm, depending on the properties of the material and the particular specification on the uniformity of the filter.
It would be desirable, therefore, if systems and methods were available for manufacturing VBG filters with increased clear aperture, increased center wavelength tuning range and improved efficiency of fabrication.